Objectives Explain how worms differ from cnidarians. Identify and describe the three major worm phyla.
Have you ever wondered why worms leave their homes in the soil? The rain floods their underground burrows. They escape drowning by crawling to the surface.
Characteristics of Worms Worms differ in two important ways from cnidarians. First, they have bilateral symmetry and a definite head. Nerves and sense organs are concentrated in the head. Second, instead of just two layers of tissue in the embryo, they have three. And the middle layer of tissue, called the mesoderm (MEHS oh DURM), forms true muscles and other tissues.
With this added complexity, worms can live in a greater variety of habitats than cnidarians. Like cnidarian medusas, some worms swim. Like polyps, some worms attach themselves to one place. But worms also burrow in mud, crawl on surfaces, and live in other organisms. And some, like the earthworm, can live in soil.
Scientists place worms in several main groups, or phyla. A worm's body structure determines which phylum it belongs to. The worm phyla with the most members are flatworms, roundworms, and segmented worms. Which group do you think earthworms belong to?
Flatworms Worms with a flattened shape are called flatworms. Flatworms have the simplest body organization of any worm. Yet, they have developed organs and organ systems. Their embryos have three layers of tissue.
Like cnidarians, their digestive cavity has only one opening. Flatworms are either free-living or parasitic. A common fresh water flatworm, the planarian, is an example of a free-living flatworm. Most kinds of planaria are only a few centimeters long. They eat tiny animals in the water and are harmless to humans.
Look at Figure 15.12 (shown below). It shows the structure of a planarian. The eyespots on the top of the head do not form images. Instead, they sense light. The nerve cells in the head form a simple brain. Food enters through the mouth, and undigested food leaves through the same opening.
A single planarian is both male and female, but each planarian must have its eggs fertilized by another. Planaria also reproduce asexually by dividing their bodies in two. Each half then grows back the missing part. If you cut a planarian into several pieces, each piece will regenerate into a new worm.
There are two groups of parasitic flatworms: flukes and tapeworms. Flukes mainly live in the body tissues of animals, including humans, and cause sickness.
Most tapeworms live in the intestines of humans and other vertebrates. The tapeworm drops pieces of its body, full of fertilized eggs, into its host's waste. If another animal eats the tapeworm pieces, the eggs will hatch inside the animal's body. Tapeworms have no digestive cavity because they absorb the digested food of their host.
Roundworms Go outside and pick up a handful of moist garden soil. What are you holding? In addition to the soil, you may have in your hand about one million tiny worms. These worms have round bodies and belong to the roundworm group, also called nematodes (NEHM uh TOHDS).
Besides their round shape, roundworms have something no flatworm has: a oneway digestive tube with two openings. Food enters a roundworm through the mouth located at the head. As it passes through the digestive tube, it is digested and then absorbed.
Undigested food leaves the tube through the anus. In roundworms, different parts of the gut can have different functions because food moves in only one direction.
Roundworms reproduce sexually. Unlike many flatworms, the sexes are sepparate. In some parasitic species, females are larger than the males and can produce over 100,000 eggs in a day.
Many roundworm species attack the roots of plants. Some of their favorite plants are crops grown by humans. On the other hand, some roundworms benefit crops by killing insect pests.
About 50 kinds of roundworms are parasites in humans. Hookworms enter a human host by boring through the skin of the feet. Using the bloodstream, they move to the lungs. They are coughed up and then swallowed.
In the intestines, they attach themselves and feed on digested food. When they have grown into adult worms, they reproduce and send eggs out with the host's waste to start the cycle over. Another roundworm parasite is Trichinella, which causes trichinosis. Trichinella eggs can enter your body if you eat undercooked pork.